1. Field of the Invention
The present invention relates to a regulated high-voltage power supply. More specifically, it relates to a regulated high-voltage power supply which comprises a tuned-collector type oscillation circuit to obtain stabilized high-voltage output by controlling the base currents for oscillation transistors of the oscillation circuit.
2. Description of the Prior Art
FIG. 1 is an electric circuit diagram showing a regulated high-voltage power supply proposed in Japanese Patent Application No. 6182/1983 (Japanese Patent Laying-Open Gazette No. 132776/1984 published on July 30, 1984).
Referring to FIG. 1, the high-voltage power supply 10 comprises a tuned-collector/grounded-emitter type oscillation circuit 3 including a regenerative capacitor C2, an oscillation transistor TR4 and the like and a control circuit 2 for controlling the base current for the oscillation transistor TR4. The high-voltage power supply 10 further comprises a resistance circuit 4 for supplying the base current to the oscillation transistor TR4 in response to the output from the control circuit 2 and a protection circuit 5 having a protective transistor TR3 which partially receives the output from the resistance circuit 4 for attenuating the output of the control circuit 2 when the output is shorted in the area of a load RL.
The resistance circuit 4 includes a first resistance circuit part 41 having a high resistance value and a second resistance circuit part 42 including a zener diode ZD. The first resistance circuit part 41 is formed by a plurality of series-connected resistors R3 and R4, the junction P2 between which is connected to the base of the protective transistor TR3 of the protection circuit 5. A negative feedback control circuit 6 is connected to the protective transistor TR3. This negative feedback control circuit 6 is provided for negative feedback of the output from a detecting resistor R9 which is connected to the high-voltage output area as shown by arrows A, thereby to control the operation of the control circuit 2. An output circuit 7 is connected to a high-voltage transformer T. This output circuit 7 is adapted to supply high-voltage output to the load RL in response to the output from the oscillation circuit 3. An input terminal V.sub.in receives a current from a DC power supply and an output terminal V.sub.out generates high-voltage output. As hereinabove described, the load RL is connected to the output terminal V.sub.out.
The oscillation circuit 3 further includes an oscillation stabilizing resistor R7, a positive feedback winding L3, a tuning capacitor C3 and a primary (low-voltage) winding L1 of the high-voltage transformer T. The first resistance circuit part 41 of the resistance circuit 4 is not mainly directed to supply the base current to the oscillation transistor TR4, but is adapted to form a short detecting circuit for detecting whether or not the high-voltage output is shorted. Therefore, the resistors R3 and R4 are set at high resistance values. The second resistance circuit part 42 of the resistance circuit 4 forms a main base-current network for mainly supplying the said base current, and the resistor R5 thereof is set at a low resistance value.
The negative feedback control circuit 6 comprises a comparison amplifier AMP a capacitor C1, resistor R1 and a reference power supply E1. A negative-phase input terminal (-) of the comparison amplifier AMP receives the output from the detecting resistor R9 as shown by arrows A while a positive-phase input terminal (+) thereof receives the output from the reference power supply E1. The output circuit 7 includes a secondary (high-voltage) winding L2 of the high-voltage transformer T, a rectifier diode D1, a smoothing capacitor C4 and a spark discharge preventing resistor R8. In addition, the regulated high-voltage power supply circuit 10 includes a delay capacitor C5 which is provided between the junction P2 of the resistors R3 and R4 forming the first resistance circuit part 41 and a grounded point P4 for delaying the rise time of operation voltage Vb of the protective transistor TR3 at the junction P2 with respect to the rise time of output voltage Va from the control circuit 2.
Description is now made on the operation of the regulated high-voltage power supply circuit 10 as shown in FIG. 1. When the power supply circuit 10 is activated, the negative-phase input terminal (-) of the comparison amplifier AMP in the negative feedback control circuit 6 is at a low input level. Therefore, the output of the negative feedback control circuit 6 rises toward a high level, whereby the output voltage Va at an output point P1 of the control circuit 2 is increased. The rise speed of the operation voltage Vb at the junction P2 is slower than that of the output voltage Va, through the function of the aforementioned delay capacitor C5.
Therefore, the output voltage Va rises to a level for making the zener diode ZD conduct before the operation voltage Vb at the junction P2 is increased to make the protective transistor TR3 conduct. Consequently, the base of the oscillation transistor TR4 is supplied with the base current through the zener diode ZD and the resistor R5, whereby the oscillation circuit 3 rapidly enters an oscillating state. Thus, when the oscillation circuit 3 enters a normal oscillating state, the oscillation transistor TR4 is continuously supplied with the base current since the output voltage Va at the output point P1 of the control circuit 2 is set to be higher than the zener voltage of the zener diode ZD. Further, the operation voltage Vb at the junction P2 is set to be lower than the driving voltage for the protective transistor TR3, so that the protective transistor TR3 is not operated.
When the high-voltage output is shorted, the oscillation is so attenuated that voltage Vc at the junction P3 between the regenerative capacitor C2 and the oscillation stabilizing resistor R7 is increased, whereby the operation voltage Vb at the junction P2 is also increased. As the result, the protective transistor TR3 conducts to decrease the output voltage Va of the control circuit 2. The decreased output voltage Va is set to be lower than the zener voltage Vz of the zener diode ZD. Therefore, the zener diode ZD enters a non-conducting state and substantially no base current is supplied to the oscillation transistor TR4, whereby the oscillation circuit 3 stops the oscillation. In addition, although a small base current still flows through the first resistance circuit part 41 to the oscillation transistor TR4, the oscillation will not be developed because of the shorting of the high-voltage output. Thus, the high-voltage power supply 10 is protected against shorting of the high-voltage output.
When the shorting of the high-voltage output is released, the oscillation circuit 3 starts a small amplitude oscillation due to the small base current flowing through the first resistance circuit part 41, whereby the voltage Vc of the regenerative capacitor C2 starts lowering and the protective transistor TR3 enters a non-conducting state. Since the rise speed of the operation voltage Vb at the junction P2 is slower than that of the output voltage Va of the control circuit 2 as hereinabove described, the protective transistor TR3 is not operated thereafter and the zener diode ZD is made to conduct. Consequently, the oscillation transistor TR4 is supplied with the required base current, whereby the oscillation circuit 3 smoothly re-starts oscillation, so that the high-voltage power supply circuit 10 may be restored to a state for supplying prescribed high-voltage output.
However, the conventional high-voltage power supply circuit 10 as shown in FIG. 1 has the following disadvantage: The storage charge in the delay capacitor C5 is so small in the conducting state of the protective transistor TR3 that the charge is rapidly discharged, whereby the protective transistor TR3 enters a non-conducting state from a conducting state in a short period of time. Therefore, when an overcurrent state caused by shorting of the high-voltage output is released, the oscillation circuit 3 is rapidly shifted to a normal oscillating state from a low-level oscillating state so that the high-voltage power supply circuit 10 is immediately restored to the high-voltage output supplying state, to again cause an overcurrent state by shorting of the high-voltage output. In other words, the overcurrent per unit time is increased, to cause danger of disconnection or fire.
When the high-voltage power supply circuit 10 is applied to a copying machine or the like, the aforementioned disadvantage leads to the following serious problem: In the copying machine, a corona discharge caused by the high-voltage output from the output circuit 7 is employed for copying operations. Such corona discharge changes into spark discharge when copying papers are jammed so as to cause shorting in a load area. Such spark discharge causes a state similar to shorting of the load area.
Although this shorting is immediately released by stoppage of oscillation based on conduction of the protective transistor TR3, the protective transistor TR3 is immediately restored to a non-conducting state and the oscillation circuit 3 re-starts oscillation since the storage charge in the delay capacitor C5 is small, whereby spark discharge again takes place. Such spark discharge is intermittently caused when the papers are continuously jammed, whereby discharge energy of the spark discharge per unit time is increased to cause paper burning, leading to danger of fire.
Further, peripheral circuits around the high-voltage power supply circuit 10 may be damaged depending on arrangement thereof. In the conventional high-voltage power supply circuit 10 as shown in FIG. 1, on the other hand, the oscillation circuit 3 is merely provided with a single oscillation circuit TR4, which conducts only for a halfwave period of the AC waveform induced by the high-voltage transformer T, whereby the output voltage of the output circuit 7 is made small. The oscillation circuit 3 is provided with the single transistor in order to prevent the aforementioned danger of fire, whereas fire cannot be completely prevented and the copying function based on normal corona discharge itself is lowered.